Just an idea of the crazy prices some seedlings can go for. Dude wants $50 for this. LOL
Funny thing is I'd pay it too! If I had the cash.
Anyway, hope you do find something good.
https://www.ebay.com.au/itm/RARE-Trichocereus-WHIORA-X-MB-SCOPULICOLA-cactus-plant/163595799784?hash=item261711cce8:g:oVAAAOSwO2pciaMt

Some kind of slime mold I reckon. Prolly not too much to worry about, but I reckon it might happily eat cacti spines. Definitely hit it with some kinda fungicide Copper should be fine. If that doesn't work try sulphur.
Love that old Portisehead song

A lot goes into getting them up to their first 10 cm. And good crosses are often sold in small lots, (with even fewer actually sprouting of course).
You'll have to pay a premium for the good stuff. That's why people usually resort to mastering the art of raising cacti from seed.
Having said all that, there may well be people willing to sell a heap for a good price etc. Hope you find the perfect plants
Not trying to throw shade at all, just putting it out there for consideration.

Agreed, the longer you leave them without water, the better. They will produce roots without getting any water anyway, so it's completely unnecessary to water them for few weeks. They'll literally go for months without water.